1. Field of the Invention
The present invention relates to a process and a device for the continuous micromixing of fluids, leading to the polymerization of monomer(s) and to the obtainment of homopolymers or copolymers.
2. Background Information
Good continuous micromixing of fluids is desired in the case of certain very rapid reactions, as well as in the case where it is desired to rapidly homogenize two or more miscible or immiscible fluids.
The process and the device according to the invention can be used in particular when the micromixing plays an important role, for example, in the yield and characteristics of the products. This is the case, in particular, for crystallization, precipitation and combustion reactions and for polymerization and copolymerization reactions.
The invention is particularly useful for ultra-fast (co)polymerizations leading to (co)polymers of controlled mass and polydispersity and, preferably, with a high solids content; this is the case, in particular, for the anionic polymerization of (meth)acrylic monomers.
The mixing of two or more fluids in a very short space of time may be necessary in certain circumstances and in particular when these fluids are reactive with each other and when the chemical reaction kinetics are complex and/or rapid. It is thus convenient, in certain cases, to mix the reactants at the molecular level (micromixing) in a time shorter than the characteristic reaction time.
To characterize the micromixing, parallel-concurrent or consecutive-concurrent type reactions can be used as test reactions.
This is the case, for example, for parallel-concurrent reactions of the type:
A+Bxe2x86x92Rxe2x80x83xe2x80x83(1)
C+Bxe2x86x92Sxe2x80x83xe2x80x83(2)
or consecutive-concurrent reactions of the type:
A+Bxe2x86x92Rxe2x80x83xe2x80x83(1xe2x80x2)
B+Rxe2x86x92Sxe2x80x83xe2x80x83(2xe2x80x2)
in which A, B and C are reactants and R and S are products.
In the parallel-concurrent reaction system, B is mixed, in stoichiometric deficit, with a mixture containing A and C. In the consecutive-concurrent reaction system, B is mixed, in stoichiometric deficit, with A. In general, R is the desired product and S is a side product. In both cases, the reaction rates (1) and (1xe2x80x2) are greater than those for the reactions (2) and (2xe2x80x2) respectively.
The proportion of R and S depends on the quality of the micromixing between A and B:
if the micromixing is good, i.e. if the characteristic micromixing time is less than the characteristic time for the reaction (2) or (2xe2x80x2), depending on the case, essentially only R will be formed;
on the contrary, if the micromixing is poor, i.e. if the characteristic micromixing time is greater than the characteristic time for the reaction (2) or (2xe2x80x2), depending on the case, S will also be formed. The amount of S formed thus depends both on the micromixing (the poorer the micromixing, the more S will be formed) and on the stoichiometry of the reactions.
In general, if S is an undesirable side product, it will be advantageous to promote good micromixing in order to increase the yield of R and, in this way, to reduce the costs of separation between R and S and to avoid the formation of a side product S which cannot be viably upgraded.
In the case of living polymerization reactions, i.e. a polymerization, in particular one which is free or virtually free of termination and/or transfer reactions, micromixing makes it possible to control the molar mass distribution. The reason for this is that one of the particular features of this type of polymerization lies in the fact that it is possible to obtain very narrow molar mass distributions, i.e. all of the macromolecular chains contain virtually the same number of monomer units. However, this condition is satisfied only if, on the one hand, the initiation reaction takes place rapidly before the propagation reactions have begun, and, on the other hand, the chains grow simultaneously. Initiation systems allow the first condition of rapid initiation to be satisfied. On the other hand, only good micromixing between the initiator and the monomer will allow the simultaneous growth of the macromolecular chains. If the micromixing is not good, some chains will start to grow before others, which, in the end, will result in broadening of the molar mass distribution.
Currently, one of the techniques most commonly used for mixing two or more liquids consists in using a closed, semi-closed or open tank, fitted with a mechanical stirrer such as a propeller or turbomixer or the like, and in injecting one or more of the reactants into the tank.
The mixing can be carried out by virtue of the energy dissipated by the mechanical stirring. Unfortunately, these devices do not make it possible, in certain cases, to achieve micromixing times that are short enough to carry out rapid and complex reactions, and above all, they are unsuitable in the case of polymerization reactions in which the viscosity increases rapidly over time.
Static mixers, placed in line in a conduit or at a reactor inlet, allow good mixing of liquids. However, they are usually used as premixers before the reactor inlet or when the time constraints are not prohibitive. They are good devices for homogenizing solutions, but are not really suitable for certain polymerization reactions, in particular rapid reactions, since there are considerable risks of blockage. This is the case, in particular, for polymerizations at a high solids content.
Tangential-jet mixers (which can be used in particular for anionic polymerization as described in EP-A-0,749,987) or RIM (reaction injection moulding) heads are confined-jet mixers, i.e. mixers involving jets which are in contact with the wall of the mixer. They are highly effective, but result in blockages when high polymer contents are used or require the injection of products by pumps which can withstand high pressures (several hundred bar). Furthermore, RIM heads require a batchwise operation.
Free-impinging-jet mixing (i.e. mixing without jets coming into contact with the walls of the mixer) is known, and has been described for creating emulsions or in liquid-liquid extraction processes, for example by Abraham Tamir, xe2x80x9cImpinging-Stream-Reactors. Fundamentals and Applicationsxe2x80x9d, Chap. 12: Liquid-Liquid Processes, Elsevier (1994).
Free-impinging-jet devices have also been described for precipitation or polymerization. They consist of two jets oriented at a given angle and whose impingement results in a rapid micromixing; cf. Amarjit J. Mahajan and Donald J. Kirwan xe2x80x9cMicromixing Effects in a Two Impinging-Jets Precipitator, Aiche Journal, Vol. 42, No. 7, pages 1801-1814 (July 1996); Tadashi Yamaguchi, Masayuki Nozawa, Narito Ishiga and Akihiko Egastira xe2x80x9cA Novel Polymerization Process by Means of Impinging Jetsxe2x80x9d, Die Angewandte Makromolekulare Chemie 85 (1980) 197-199 (No. 1311). The drawback of these systems is that they only allow two fluids to be mixed and that the jets are all of the same diameter and, consequently, if it is desired for the mixing to be efficient, the respective flow rates in each jet must all be equal to each other. In the case of a polymerization reaction, since the monomer arrives in a first jet and the initiator solution arrives in a second jet at the same flow rate as the first, it is thus seen that the amount of solvent in the system will necessarily be relatively large, which means having to involve recycling operations, that are generally expensive, downstream of the polymerization process.
The subject of the present invention is thus a polymerization process comprising continuous free-impinging-jet micromixing, which no longer incurs the limitations which have just been described, as well as a device for carrying out this process.
The process according to the invention, for the continuous preparation of homopolymers or copolymers by free-impinging-jet micromixing of fluids formed (1) of monomer(s) and (2) of an initiator system, is characterized in that the said fluids (1) and (2) are micromixed and the mixture of these fluids is recovered in the form of a resultant jet Jr, originating from the point of impingement (I), the micromixing being obtained by:
a) forming at least one group of at least two jets Ja of the said identical or different fluids, these jets coinciding at a point of impingement (I), the jets Ja of the same group all being of identical geometry, their axes being arranged such that their projections on a plane perpendicular to the axis (A) of the resultant jet Jr are distributed angularly in a uniform manner and these axes being inclined relative to the said axis (A) by the same non-zero angle xcex1 of not more than 90xc2x0;
b) simultaneously directing at the point of impingement (I) at least one jet Jb of a fluid which is different from at least one of the fluids of the said jets Ja for forming the resultant jet Jr, the axis of the jet(s) Jb being inclined relative to the axis of the resultant jet Jr by an angle xcex2 which is less than the angle xcex1; and
c) recovering the mixture of the fluids in the jets Ja and Jb in the form of the resultant jet Jr consisting of the homopolymers or copolymers.
The jets Ja and Jb are preferably arranged such that the resultant jet Jr, obtained from the point of impingement (I), has a vertical direction directed downwards.
The term xe2x80x9cgeometryxe2x80x9d is understood to refer to the shape of the jets Ja and Jb, which can be in cylindrical, conical, sheet, etc. shape. Preferably, the jets are cylindrical and have a cross-sectional diameter of, for example, between 0.01 mm and 100 mm, and preferably between 0.1 mm and 10 mm. The jets Ja and Jb can have the same geometrical shape.
The term point of impingement is understood to refer to the initial zone of contact between all of the jets.
The angle xcex1 is preferably from 10 to 60xc2x0; the angle xcex2 is from 0 to 89xc2x0. The angle xcex2 is preferably 0xc2x0. The direction of the single jet Jb then joins together with that of the resultant jet Jr.